By Eloise Gibson
Pinus Radiata sequesters carbon at a much higher rate in NZ than much-preferred native trees. So scientists propose an unconventional solution to get the best of both.
To measure how much carbon is in a tree, you first have to kill it.
You slice up the trunk, branches, twigs, leaves and roots and dry the dismembered tree parts in an oven. Then you weigh them.
“It takes a long time,” says Euan Mason, a professor at the University of Canterbury’s School of Forestry. “I did some in 2012 with two students, and in six weeks I think we did 25 trees.”
Sacrificing trees like this is expensive, but researchers need these measurements.
Typically, about half a tree’s dry weight is carbon, which you can multiply by roughly 3.7 to work out how much carbon dioxide the tree has sucked from the atmosphere.
Once enough trees of different ages and species have been dissected, the results are used to help build computer models estimating how much carbon is in a hectare of living forest, or an entire country’s worth of trees.
Forest owners can use models like this to see how much money they can claim for carbon credits under the Emissions Trading Scheme. Similar estimates tell the Ministry for the Environment that New Zealand’s forests removed 24 million tonnes of carbon dioxide equivalent from the atmosphere in 2017, enough to offset 29 percent of the country’s greenhouse gas emissions.
Most of this CO2 was absorbed by Pinus Radiata, a species much-loved by commercial foresters for its astonishing rate of growth, but seemingly little-loved by anyone in the general population.
Radiata became the nation’s wood crop after most of our ancient Kauri forests were destroyed by indiscriminate logging in the 1880s. (“I wouldn’t call it forestry, because it was just pillaging,” says Mason).
Permanent indigenous forest still covers a much larger area than pine – almost quarter of the country, compared with 6.6 percent in wood plantations. But old-growth forests on conservation land are excluded from the tallies of New Zealand’s carbon sinks and emissions. (This sounds less insane after you find out that mature forests often reach a steady state, sucking about the same amount of CO2 they are losing from dead wood.)
For such peaceful beings, trees have sparked some heated arguments lately: how many we should plant, where and what kind. One point on which no one disagrees is that New Zealand needs to hold on to its old, indigenous forests: mature forest in the conservation estate holds about twice as much carbon per hectare as tree plantations do. After all, our ancient forest has centuries to hoard it.
But the question of what to plant in the next few decades is different, and even forestry scientists can’t agree. The basic points are common ground. We face a climate emergency. The Government, like others around the world, is committed to being carbon neutral by 2050. Trees can help.
But do we want maximum carbon-sucking, fast, or do we value other attributes more, or is there some way to have it all?